The present invention relates to a flip-flop formed by semiconductor devices, and more particularly, to a flip-flop adapted to be formed in a semiconductor integrated circuit.
Among flip-flops, a set-reset type flip-flop (R-S flip-flop), a trigger type flip-flop (T flip-flop), a delay type flip-flop (D flip-flop), a J-K flip-flop, etc. are known. In the following, the present invention will be described in connection with T flip-flops as an example. The present invention is also applicable to other types of the flip-flops.
A T flip-flop is a flip-flop whose output is inverted in response to rising or falling of an input signal, and it is frequently used in a frequency-divider, a counter, or the like. It is also often used in an analog integrated circuit such as, for example, a D/A converter, a demodulator for a PAL-system television signal, an FM stereo, and other frequency- or phase-control circuits.
In a known T flip-flop suited for such use, a base of one of two transistors whose emitters are connected in common is connected via a resistor to the collector of the other transistor, and the base of the other transistor is connected via a resistor to the collector of the one transistor. The collectors of the respective transistors are connected via respective resistors to a power supply line. A trigger input signal is applied to the bases of the above-mentioned two transistors via an input circuit comprising two resistors and another pair of transistors. The collectors of the latter pair of transistors are connected to the bases of the other transistors in the same pair. The trigger input signal is applied to respective collectors of the transistors in the latter pair via resistors, respectively, and then applied from the respective emitters of the transistors in the latter pair to the respective bases of the transistors in the former pair, respectively. An output signal is derived from a collector of one of the first-mentioned two transistors and applied to the base of an output transistor via a resistor.
As described above, because many resistors were used in the heretofore known T flip-flop, the conventional flip-flop occupied a large area in a semiconductor integrated circuit. Especially for the purpose of reducing power consumption, the resistances of the respective resistors were designed to be as high as possible. A resistor having a high resistance, however, also occupies a large area. Consequently, such reduction of power consumption limited the amount of reduction of the occupation areas of the respective resistors. Moreover, since a diffused resistor was used in a bipolar integrated circuit for each resistor, the resistance value of the diffused resistor was restricted to a maximum of about 50 K.OMEGA.. Hence, there was the shortcoming that current consumption of the entire T flip-flop was not sufficiently reduced. Furthermore, while the T flip-flops in the prior art were triggered in response to a voltage, in many cases the trigger signal to be applied is a current signal. Consequently, the trigger signal was applied after the current signal was converted into a voltage signal. This current-to-voltage conversion was normally performed by a converter which was formed of resistors or formed of transistors and resistors and which was required in addition to the T flip-flop. Therefore, the occupation area of the T flip-flop was further increased. Especially, the resistor in this current-voltage converter required a large resistance value. In this respect also, it was difficult to reduce the occupation area of a T flip-flop in semiconductor integrated circuits.